Electronic irrigation controllers typically include a microprocessor or micro-controller that executes a stored watering program to turn a plurality of solenoid actuated valves ON and OFF. The valves deliver water to sprinklers that supply water to turf and/or other landscaping. Electro-mechanical rain sensors are often connected to irrigation controllers to suspend watering during periods of rainfall.
In a first type of conventional rain sensor, a bucket collects a certain amount of rain water, and then automatically tips so that the bucket is emptied. The process of filling and emptying the bucket continues as long as it is raining. The bucket is typically calibrated to empty after a predetermined amount of rain, for instance, one-tenth of an inch. A set of switch contacts close every time the bucket tips, generating one pulse for every one-tenth of an inch of rainfall. By way of example, one and one-half inches of rain would be indicated by fifteen pulses supplied by the rain bucket switch.
In a second type of conventional rain sensor, rain drops are directed to a common location where they are individually counted. This is often done by directing water droplets over closely spaced apart electrodes and detecting a conductive path through the individual water droplets. In this type of rain sensor one droplet is represented by one pulse. The size of the receptacle that catches the rain drops results in a certain number of droplets equating to a predetermined amount of rainfall.
In a third type of sensor a plurality of discs made of a hygroscopic material are mounted in a housing that collects rainfall and deposits it on the hygroscopic material through slots or other apertures formed in the housing. The discs expand as they absorb moisture, causing a two-state electro-mechanical switch to change states once the discs have generated a predetermined amount of movement via physical expansion of the same. The third type of rain sensor does not directly measure a given amount of rain however it is highly reliable and its simplicity and lower cost has resulted in its successful commercialization over many years. See U.S. Pat. No. 3,808,385 of Klinefelter granted Apr. 30, 1974.